1. Field of the Invention
This invention relates generally to document scanning apparatus for placing a line-by-line image of a stationary document on a linear photosensitive surface.
2. Description of the Prior Art
In high speed document scanning, it is often desirable to keep the document being scanned stationary because of the problems associated with high speed paper handling. While in some copying apparatus, an image of an entire document may be flashed onto a photoresponsive surface at once, it is preferable in other instances to page scan the stationary document a line at a time. Such instances include those wherein power concern makes it impractical to illuminate the entire document at once, where a linear photosensitive array is provided as the image receptor, where the printer operates a line at a time and/or where the image is to be converted into a series of electrical signals. In prior art devices for "page scanning" stationary documents (i.e. scanning documents a line at a time), a lens, a mirror or another optical element is moved through a path such that a flowing line image of the stationary document is produced at a photosensitive receptor. The optical element projects successive line segments onto the receptor, which may be a rotating drum as in U.S. Pat. No. 3,528,738, moving web as in U.S. Pat. No. Re. 28,016 or stationary array of photosensors as in co-assigned U.S. Patent Application Ser. No. 740,699 filed Nov. 11, 1976 in the name of John H. Ladd, now U.S. Pat. No. 4,058,828.
In such apparatus having moving scanning elements, after each image has been projected upon the photosensitive surface, the scanning elements must return to their original starting positions before the next cycle. This limits the speed of the overall copy cycle by necessitating that a "return period" be provided for the elements. If an attempt is made to decrease the copy cycle period by increasing the return speed, high levels of acceleration and deceleration, accompanied by excessive forces, wear and vibration, are soon encountered.
In co-assigned U.S. Pat. No. 3,528,738 which issued Sept. 15, 1970 to H.T. Hodges, these detrimental effects were solved, insofar as they resulted from returning the scanning lights for slit illumination to their starting position, by providing multiple light units which always move in the same direction around a closed path. However, the scanning optics were unmodified from earlier copiers and still required a period for return motion.
That return period was utilized in the scanning system disclosed in U.S. Pat. No. 3,524,928 which issued to T. A. Hoskins on Aug. 18, 1970. Hoskins discloses a document scanner comprising two distinct optical systems. As one optical system moves in a direction to page scan the document, the other optical system moves to a position to commence a second page scan as soon as the first optical system completes its page scan. While the first optical system returns to its initial position, the second optical system scans the document. By this arrangement, there is no time wasted between successive page scans. While the speed of the Hoskins system is not limited to the return period of its scanning optics, it requires two separate optical systems, the movements of which must be synchronized. As may be readily appreciated, this adds considerably to the cost of the system.